1. Field of the Invention
The present invention relates to a threading/tapping control apparatus for synchronous control of a rotational shaft and a linear feed shaft when threading or tapping a workpiece.
2. Description of the Related Art
When threading a workpiece, the threading is carried out by rotating the workpiece while using a cutting tool to give a predetermined cut to the workpiece and making the cutting tool move relatively in the workpiece axial direction. In this case, the cutting tool must be made to move relatively with respect to the workpiece in the axial direction in synchronization with the rotation of the workpiece. Further, when tapping a workpiece by a tapper, the rotation of the tapper and the feed in the axial direction must be synchronized.
In this threading, to synchronize the rotational shaft and the linear feed shaft, the method of detecting the rotational speed of the rotational shaft (spindle) rotating the workpiece, finding a movement command of the feed shaft of the tool from this detected rotational speed, and making the tool move in the axial direction by it so as to make the tool move in the axial direction in synchronization with the rotation of the workpiece to perform the threading is known (see Japanese Unexamined Patent Publication (Kokai) No. 5-69275).
Further, an invention outputting the movement command to a motor of a feed shaft for feeding a tapper in the axial direction and performing synchronous control using the output of a pulse generator attached to the motor and the acceleration of the movement command of the tapper feed shaft added together as the rotational speed command of the tapper is known (see Japanese Unexamined Patent Publication (Kokai) No. 8-141839).
Summarizing the problems to be solved by the invention, when threading/tapping a workpiece, synchronization between the rotational shaft and the feed shaft must be obtained. In the past, the speed of one shaft is made constant, and the other shaft is driven in synchronization with the speed of that shaft in the machining. For this purpose, the movement speed of one shaft of the rotational shaft or the feed shaft is detected, and the speed of the other shaft is controlled so as to be synchronized with that speed.
If, however, in threading, the operating speed changes due to a change of the command or an outside disturbance, there is the problem that synchronization error occurs between the workpiece and the tool and the threading precision is degraded. Further, in the case of tapping, if the delay of a servo system of an acceleration/deceleration zone becomes large, the synchronization error will become large and faster machining will become difficult.